First: There seems to be a bit of a disconnect in how people are looking at voting. They're focusing on whether or not groups will vote against Obama in the general by looking at whether they're voting against him in the primary.
But that's wrong. People do vote against candidates, but they also vote for them. Anybody who voted for Hillary and not against Obama will likely vote Obama in the fall. And anybody who did vote against Obama may well vote against McCain in the fall, as they dislike McCain more than Obama.
That's why you can't claim that the primary results show that Obama is alienating anybody, as opposed to Clinton attracting them for whatever reason. The basic mechanics of voting simply don't work like that.
Second: I don't want to get all postmodernist, but the American media is Othering the HELL out of blacks at this point. Everybody's looking at working-class blue-collar white men and whether or not they're likely to support Obama in the primary in, say, Indiana... but North Carolina's blacks are completely written off in the coverage. "Well, they're black, we don't need to even pay attention to their votes" is the order of the day. This despite the fact that any Democratic candidate is going to need their votes more so than the vanishingly small number of true swing-voters that fit that blue-collar male demographic.
(If Republicans didn't attract blue-collar males who weren't already Democratic for economic reasons, there wouldn't be a Republican party. Obama's would have a hard time erasing that advantage, but Hillary "Hillarycare" Clinton wouldn't have a prayer. Sorry, but she wouldn't.